Sylvester Stallone is about to score a massive payday after putting his watch collection up for auction, because one of the timepieces could fetch a literal king's ransom.

As you might have heard, the 'Rocky' star is selling 11 watches from his personal collection through Sotheby's next month ... and one crown jewel, in particular, has a watch expert going bonkers 'cause it's incredibly rare (and damn expensive).

YouTube personality Nico Leonard van der Horst -- who knows all things watches -- tells TMZ ... he simply can't believe Sly is parting with his ultra-rare Patek Philippe Grandmaster Chime.

To us normies, it's just a watch, but to van der Horst and co. ... it's the "Holy Grail."

The Grandmaster Chime is one of the most complicated and exclusive wristwatches made -- with just a small handful in circulation -- and that's why Leonard says it'll go for well over $5 million, which is what it's pegged to fetch once it actually goes up for sale.

The last time a Patek Philippe Grandmaster Chime was up for auction, it sold for a record $31 million. That was in 2019, and that one wasn't owned by Rambo, which is why Leonard tells us this one from Sly will likely go for far more than the $5 mil estimate.

As for what's so special about it ... he gives us a sense of what it can do, and why it's a collector's item -- and, apparently, it all has to do with functionality, anywhere in the world.

The other 10 watches include a separate Patek Philippe model expected to nab six figures, along with a 60th anniversary Rolex and several Panerai watches, one which Sly donned in 1996's “Daylight."

All in all, he'll be cleaning up big time, especially for the Chime.